1. Field of the Invention
This disclosure relates to the field of flow measurement, and in particular to apparatus and processes for measuring flow using nuclear or electron magnetic resonance.
2. Description of the Prior Art
The idea of studying flow by magnetic resonance dates back to the work of the early pioneers as described, for example, in Mansfield, P; Morris, P. G.; “NMR Imaging in Biomedicine”; Advances in Magnetic Resonance, Supplement 2; 1982; Academic Press, Inc. Orlando 32887; p. 235 section 7.3.5. Prior art devices for flow measurement or flow mapping rely on two well-known methods viz. “Time-of-Flight” of saturated or unsaturated spins or “Phase-Encoding” by application of a gradient field along the direction of flow. (Cho, Z. et. al.; “Foundations of Medical Imaging;” John Wiley & Sons, Inc., New York, 1993, p 374-386.) Exemplary of the “Time-of-Flight” method is U.S. Pat. No. 4,782,295 to Lew and of the “Phase-Encoding” method is U.S. Pat. No. 5,532,593 to Maneval. Analysis of chemical composition by chemical shift is discussed in “Principles of Magnetic Resonance,” third edition chapter 4, by Slichter, C. P., Springer-Verlag, N. Y. 1989. The nuclear Overhauser effect is discussed in chapter 7.
Those prior art methods employ pulse techniques that broaden the bandwidth thereby increasing the noise in the measurement signal. A departure from this prior technology is the quasi-steady-state technique in U.S. Pat. No. 6,452,390, incorporated by reference herein for all purposes, in which simultaneous spatial and temporal nutation and phase encoding of the moving spins permit simultaneous reception of the measurement signal in the continuous presence of the adjustable Larmor frequency excitation field. The quasi-steady-state condition creates a minimal bandwidth, limiting the Johnson Nyquist noise in the received measurement signal. Further, prior art methods pertain to measurements within a conduit and not to measurements around the device where there may be a strong component of velocity peripheral to the device.